Studies from our laboratory have focussed on the molecular and immunological properties of human T-cell lymphotropic virus type II. Employing virus isolates from our patient population we have demonstrated that there are in fact three molecular subtypes of the virus which have now been designated HTLV-IIa, IIb and IIc. Preliminary studies have also suggested that these may be phenotypic differences between the three subtypes. Specifically it could be shown that the tax protein which is believed to be important in HTLV leukemogensis differs between the subtypes and current studies are underway to determine if this may result in idfferences in subtype pathogenicity. In our immunological studies we have focussed on the tropism of HTLV-II infection in vivo and investigated whether infection porduces clonal expansion of the infected cell populations. These studies have shown that all three virus subtypes have a preferential tropism for CD8+ T lymphocytes. In addition the infection does appear to cause expansion of these. Studies underway and in the future will determine the role of virus regulatory proteins in this process, and the importance of expansion as an early event in virus- induced leukemogenesis.